Cellular telephone communications have rapidly grown in popularity within the United States. Originally the Federal Communications Commission (FCC) allocated cellular communications over the frequencies of 825-845 megahertz for receive, and 870-890 megahertz for transmit, with a channel bandwidth of 30 kilohertz and a transmit-receive separation of 45 megahertz. Both the transmit and receive bands were originally divided into 10 megahertz sub-bands designated for wireline and non-wireline providers respectively. The wireline service is typically the regional telephone company providing service in a given location while the non-wireline service is provided by any private entrepreneur who through licensing procedures, has been allocated a particular geographic area for cellular communications.
Originally the cellular receive band was divided into two sub-bands 825-835 megahertz for non-wireline and 835-845 megahertz for wireline providers. Within a few years after this allocation of frequencies, it became apparent that more frequency spectrum was required due to the popularity of cellular communications. As a result, the Federal Communication Commission increased the overall receive bandwidth to 824-849 megahertz and the transmit bandwidth to 869-894 megahertz. Due to this expansion of frequencies, the non-wireline receive sub-band was made into two sub-bands, one at 824-835 megahertz and a second at 845-846.5 megahertz. For wireline services, two receive sub-bands were also established, one at the original 835-845 megahertz and a second at 846.5-849 megahertz. Similar dual sub-bands for both non-wireline and wireline services were also established for the transmit band (869-880 megahertz and 890-891.5 megahertz for non-wireline services and 880-890 megahertz and 891.5-893 megahertz for wireline services). Such split frequency allocations have greatly complicated the filtering necessary for cellular communications. The dielectric notch resonator of the present invention addresses this problem by providing a high quality factor (high Q) resonator which through its associated coupling reactance component, effectively presents a low impedance throughout a narrow bandwidth at a desired center frequency so as to be particularly suited for use in notch filter applications.
Although dielectric resonators are well known in the art, the present invention also employs a coupling reactance for generating a low impedance over a narrow bandwidth and for adjusting the symmetry of this bandwidth so as to achieve a uniform low impedance notch to effectively suppress unwanted frequencies.